1. Field of the Invention
The present invention relates to a microgyroscope for detecting an inertial angular velocity of an object and a method for manufacturing the same, and more particularly, to a high-vacuum packaged microgyroscope in which a suspension structure is vacuum packaged at the wafer level and signal processing circuitry integrated therein can be interconnected in the form of a flip chip to external circuitry, and a method for manufacturing the same.
2. Description of the Related Art
FIG. 1 illustrates a conventional integrated micro pressure sensor manufactured by anodic bonding. The integrated micro pressure sensor in FIG. 1 includes a first glass plate 1 as an anodic bonding frame, a silicon substrate 2, a first p+-layer 3 as a vibrating plate for sensing a pressure variation, a second p+-layer 4 as an electrode for measuring a reference electrostatic capacitance, a first metal electrode 5 for sensing an electrostatic capacitance change, a second metal electrode 6 for measuring a reference electrostatic capacitance, an ASIC circuitry area 7 for processing a variety of signals, a getter 8 for adsorbing gases to decrease the inner pressure near to vacuum levels, a conductive epoxy resin (not shown) for interconnection to external circuitry, and a second glass plate 10 as another anodic bonding frame. The first and second glass plates 1 and 10, between which the micro pressure sensor structure formed on the silicon substrate 2 is interposed, serves as a vacuum case, and is evacuated to a pressure of 10xe2x88x926 torr, thus producing a vacuum, which allows the micro pressure sensor to operate with high accuracy. The micro pressure sensor structure includes a first capacitor structure for sensing a variable electrostatic capacitance, which includes the first p+-layer 3 formed of the silicon substrate 2 and the first metal electrode 5 deposited on the second glass plate 10, and a second capacitor structure for sensing a reference electrostatic capacitance change, which includes the second p+-layer 4 formed of the silicon substrate 2 and the second metal electrode 6 deposited on the second glass plate 10. The first p+-layer 3 of the first capacitor structure vibrates in accordance with external pressure, thus causing the gap between itself and the first metal electrode 5 to vary. Hence, the electrostatic capacitance changes depending upon the pressure applied to the first p+-layer 3. Meanwhile, the second p+-layer 4 of the second capacitor structure remains still without vibration, so that the gap between itself and the second metal electrode 6 remains constant, and thus the electrostatic capacitance does not change. Changes in the electrostatic capacitance of the first capacitor structure are measured with respect to the reference electrostatic capacitance of the second capacitor structure, thus enabling measurement of small changes in pressure. The getter 8 is a gas adsorbing material for evacuating the space between the first and second glass plates 1 and 10.
Such a micro pressure sensor operates very accurately in a strong vacuum. The same principle can be applied to microgyroscopes. In addition, microgyroscopes along with their associated signal processing circuitry must be small enough to be used in, for example, camcorders, 3D-mouses for Internet TV, automatic navigation systems and the like. This requirement is a limiting factor in most micro sensors as well as microgyroscopes. For various electrostatic capacitive sensors in which vibration of a microgyroscope enables the sensors to operate, vacuum packaging of the suspension structure is needed for decreased driving voltage of the circuit and increased sensitivity. One of the many approaches for satisfying the need has been to use glass-to-silicon bonding with an anodic bonding technique, which has been conducted primarily in the Esashi Laboratory in Douhuku University in Japan.
However, such an anodic bonding technique causes contamination of an IC circuit by sodium ions and needs an additional electric field shielding technique for protection of the IC from a high voltage applied during bonding. Therefore, the anodic bonding technique may present a fatal problem in ICs during bonding. In addition, generation of excessive oxygen at bonding sites during bonding hinders the evacuation of the structure, so that there is a room for improvement.
To solve the above problems, it is an objective of the present invention to provide a high-vacuum packaged microgyroscope and a method for manufacturing the same, in which a substrate with a signal processing ASIC circuit is mounted on another substrate including a suspension structure of a microgyroscope in the form of a 3-dimensional flip, which decreases the area of the device and the length of interconnection between circuits, and the two substrates are vacuum sealed at the wafer level at low temperatures by a co-melting reaction between a metal and silicon.
An aspect of the above object of the present invention is achieved by a high-vacuum packaged microgyroscope comprising: a first substrate including a suspension structure suspended over a groove cavity formed at the center of one surface thereof, and inner and outer electrode pads; and a second substrate including a signal processing circuit for sensing the motion of the suspension structure, an interconnect for extracting electrodes of the signal processing circuit and the suspension structure of the second substrate to the outside, and inner and outer metal/semiconductor composite layers for vacuum sealing the first and second substrates, wherein the first and second substrates are placed such that the suspension structure and the signal processing circuit thereof face each other, and then sealed by co-melting bond between the outer electrode pads and the outer metal/semiconductor composite layers, and between the inner electrode pads and the inner metal/semiconductor composite layers, to form a vacuum space in the groove cavity which receives the suspension structure, so that the first and second substrates are grounded and the electrodes of the suspension structure and the signal processing circuits are extracted to the top of the second substrate through the interconnect.
Preferably, the first and second substrates are formed of silicon. Each of the first and second substrates may further comprise a first passivation layer formed of silicon oxide or nitride to protect the first and second substrates. The suspension structure may be formed of a polysilicon or monosilicon layer. The inner and outer electrode pads of the suspension structures may be formed of a polysilicon layer, a polysilicon/gold (Au) composite layer, or polysilicon/aluminum (Al) composite layer. The interconnect may be formed on both sides and the top and bottom edges of the second substrate surrounded by the first passivation layer, the both sides of the second substrate being formed by presence of through holes, and a second passivation layer is formed on the interconnect for insulation. The outer metal/semiconductor composite layers may be formed on the outer sides of the second passivation layer. Preferably, the degree of vacuum of the groove cavity reaches down to 10xe2x88x926 torr. Preferably, the inner and outer metal/semiconductor composite layers are formed of a thin Au/Si or Al/Si composite layer, such that the inner and outer metal/semiconductor composite layers are melted and bonded with the inner and outer electrode pads at a low temperature of 363 to 400xc2x0 C. to create a vacuum space without causing the formation of voids at bonding sites.
Another aspect of the object of the present invention is achieved by a method for manufacturing a high-vacuum packaged microgyroscope, comprising the steps of: (a) etching a first substrate to form a groove cavity at the center of the first substrate, where a suspension structure is to be formed, and forming a first passivation layer for protecting the first substrate; (b) depositing a polysilicon layer on the etched surface of the first substrate and patterning the polysilicon layer into inner and outer electrode pads; (c) forming a suspension structure by depositing a sacrificial layer over the inner and outer electrode pads, patterning the sacrificial layer to form openings to be anchors for sustaining the suspension structure, depositing polysilicon over the opening and the sacrificial layer and patterning the deposited polysilicon layer; (d) removing the sacrificial layer by etching to float the suspension structure; (e) forming an oxide pattern on a second substrate having a signal processing circuit for sensing the motion of the suspension structure; (f) patterning the second substrate using the oxide pattern as an etching mask to form through holes for interconnection to the outside, removing the oxide pattern, and forming a first passivation layer for protecting the entire surface of the second substrate; (g) forming an interconnect by depositing a polysilicon layer to cover both sides and the top and bottom edges of the second substrate surrounded by the first passivation layer, and patterning the polysilicon layer; (h) depositing a second passivation layer to cover the interconnect and the second substrate and patterning the second passivation layer to form openings for interconnection to the outside; (i) forming inner metal/semiconductor composite layers for connection through the openings to the interconnect, and outer metal/semiconductor composite layer for vacuum packaging on the second passivation layer; and (j) vacuum sealing the first and second substrates by co-melting bond between the inner electrode pas of the first substrate and the inner metal/semiconductor composite layers, and between the outer electrode pads of the first substrate and the outer metal/semiconductor composite layers of the second substrate, to maintain the cavity of the suspension structure in a vacuum condition.
Preferably, in the step (b) the inner and outer electrode pads of the suspension structure are formed of a polysilicon layer, a polysilicon/gold (Au) composite layer, or a polysilicon/aluminum (Al) composite layer. Preferably, in the step (g) the polysilicon is deposited by low pressure chemical vapor deposition (LPCVD). Preferably, in the step (h) the second passivation layer is deposited by plasma enhanced chemical vapor deposition (PECVD). Preferably, in the step (i), the inner and outer metal/semiconductor composite layers are formed of a Au/Si or Al/Si composite layer. Preferably, in the step (j), the melting and bonding are performed at a low temperature of 363 to 400xc2x0 C., and the degree of vacuum in the cavity reaches down to 10xe2x88x926 torr.